Watching the Changing M87 Black Hole Event Horizon

The event horizon is a fascinating part of a black hole’s anatomy. In 2017, telescopes around the world gathered data on the event horizon surrounding the supermassive black hole at the heart of M87. This was the first time we had ever seen an image of such a phenomenon. Since then, 120,000 more images of the region have been captured and, as astronomers sift through the data, their model of M87’s event horizon has evolved. 

Black holes, formed from the collapse of massive stars or in some cases through other processes, are regions of space-time where gravity is so intense that it warps the fabric of the universe. The event horizon is the boundary surrounding a black hole, beyond which nothing—not even light—can escape its gravitational pull. It marks the point of no return for any matter or radiation that gets too close. Within the event horizon, the curvature of space-time becomes infinite, leading to a singularity, a point where density and gravity reach extremes that modern physics and mathematics struggle to model. The event horizon’s properties are critical to understanding black holes, as it represents the outermost layer hiding everything within. 

One such object sits at the centre of most galaxies and in particular at the centre of M87, a massive elliptical galaxy 53 million light years away. It’s approximately 120,000 light years across with an estimated trillion stars. At its core is a supermassive black hole which weighs in at about 6.5 billion times the mass of the Sun. It was this object which was imaged back in 2017 for the first time. 

Since that first image of the event horizon around the M87 black hole, over 120,000 images have been used to analyse how the horizon has evolved since the first images were captured. Like all black holes, M87’s has a rotational axis and it is this, that the images have revealed something unexpected. 

A team of astronomers have confirmed that the axis points away from the Earth and have shown that the accretion disk suffers turbulence. Compared to images from 2017, the accretion disk has brightened and it is thought the turbulence in the accretion disk is the cause. As assistant professor Hung-Yi Pu from National Taiwan Normal University explains “the black hole accretion environment is turbulent and dynamic. Since we can treat the 2017 and 2018 observations as independent measurements, we can constrain the black hole’s surroundings with a new perspective.”

The accretion disk around M87* (as the black hole is referred to) is a swirling disk of gas and dust that orbits around the black hole before being pulled in. The disk forms when matter is stripped off nearby stars or from interstellar gas before spiralling in to the black hole under its immense gravitational pull. As the material accelerates in the disk and gets compressed, it heats up to millions of degrees, emitting radiation across the electromagnetic spectrum. It’s this radiation that often reveals the presence of a black hole.

The discoveries from the super computer generated images reveal more about the dynamics in the regions surrounding a black hole. They find that material spiralling into a black hole from afar can either flow in the direction of the black hole’s rotation or in the opposite direction. 

Source : M87 One Year Later: Catching the Black Hole’s Turbulent Accretion Flow

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